It is well-known that electrostatic actuators and sensors may enter an undesired so-called collapsed state under certain operating conditions such as, e.g., when exposed to extraordinarily high sound pressure levels or a mechanical shock.
The collapsed state is characterized by a “collapse” or sticktion between the diaphragm and the back-plate, such as that described in PCT patent application WO 02/098166 which discloses a silicon transducer element. When a polarity of an incoming sound pressure is such that the diaphragm, usually the moveable plate, is deflected towards the back-plate, the force originating from an impinging sound pressure is combined with an attractive force originating from a DC electrical field provided between the diaphragm and the back-plate. When a sum of these forces exceeds a predetermined critical value, an opposing force provided by a diaphragm suspension will be insufficient to prevent the diaphragm from approaching and contacting the back-plate, causing the microphone to enter a collapsed state. The diaphragm can only be released from the back-plate once the attractive force originating from the DC electrical field acting on the diaphragm has been removed or at least significantly reduced in magnitude.
U.S. Pat. No. 5,870,482 discloses a silicon microphone where mechanical countermeasures have been included to prevent diaphragm collapse by restricting maximum deflection of the microphone diaphragm to less than a collapse limit which in the disclosed microphone construction is about 1 μm.
In silicon condenser microphones where no special means have been applied to prevent collapse of the diaphragm, fully or at least party removing the microphone DC bias voltage will remedy the collapsed state and secure that the transducer element returns to a normal or quiescent state of operation. Usually, the diaphragm and the back-plate condenser plates have both been treated with a non-conducting anti-sticktion coating which will prevent Van der Waal forces from keeping the diaphragm sticking even if the DC bias voltage that generates the DC electrical field between the transducer element diaphragm and back-plate has been removed (i.e., zeroed).
However, a collapse detection and control circuit adapted for use in condenser microphones has not yet been disclosed. The present invention is directed to satisfying this and other needs.